5839
tion by phenyl is vastly superior in stabilizing cyclopentadienyl ions, but not so much so in the pyramidal forms. Substitution at a base position in the pyramidal ions (X) is better than at the apex (XI). (1) and (2) may be generalized to several substituents. The more stable dimethyl IV has both substituents in basal positions (XIII) rather than one apical (XIV), consistent with the monomethyl species, but contradicting Masamune's observations7&and Hoffmann's extended Hiickel calculations. l8 Pentamethyl substitution stabilizes all forms markedly, favoring the pyramidal ion (XVI) slightly over cyclopentadienyl (XV). The effect is not nearly enough to offset the large difference in stabilities of the parent ions, and XV is still of lower energy. The separation between singlet and triplet pentamethylcyclopentadienyl cations (XV US. XVII) is smaller than that in the parents. Acknowledgments. W. J. H. thanks the Centre de Recherches, Roussel Uclaf, for a generous fellowship. The work at Princeton was supported by grants from the National Science Foundation and Hoffmann-La Roche, Nutley, N. J. (18) R. Hoffmann, private communication, cited in ref 7a. (19) Direct all correspondence to this author at the Department of Chemistry, University of California, Irvine, Calif. 92664.
Warren J. Hehre* l 8 h b o r a t o i r e de Chimie Thdorique, UniversitB de Paris-Sud and Centre EuropPeti de Cacul Atomique et Moldculaire Centre d'Orsay 91405 Orsay, Frunce
Paul v. R. Schleyer Department of Chemistry, Princeton University Princeton, New Jersey 08540 Received April 7, 1973
A One-Step Synthesis of Optically Pure l,a-Ethano Bis Sulfoxides and Phosphine Oxides via the Copper-Promoted Oxidative Dimerization of Chiral Sulfinyl and Phosphinyl Carbanions Sir: Optically active ethano-bridged disulfoxides and diphosphine dioxides are useful as ligands in coordination chemistry and as key synthetic intermediates.' We have developed a practical and versatile one-step synthesis which furnishes either enantiomer of these substances in high enantiomeric purity. The synthetic strategy consisted of assembling two subunits of these substituted ethanes, each containing a chiral center at sulfur or phosphorus, i.e., the enantiomerically pure methyl-substituted sulfoxides or phosphine oxides. Our goal was realized by the copperpromoted oxidative coupling2 of the a-carbanions derived from the monofunctional precursors, as depicted in the following scheme.
II RiMCH, 1
0
0
0
strong ---f
base
/I RIMCHzI
11 + (RIMCH2)2 salt I cu
Rz R2 Rz M = S (Rz = electron pair) or P (1) For example, see S . K . Madan, C. M. Hull, and L. J. Herman, Znorg. Chem., 7, 491 (1968); B. J. Brisdon, J . Chem. SOC., Dalfon Trans., 2247 (1972); S . S. Sandhu and R. S . Sandhu, Znorg. Chim. Acta, 6 , 383 (1972); F. Mani and M. Bacci, ibid., 6 , 487 (1972). (2) (a) T. Kauffmann and D. Berger, Chem. Ber., 101, 3022 (1968); (b) T. Kauffmann, M. Schonfelder, and J. Legler, Jusrus Liebigs Ann. Chem., 731.37 (19701, and references cited therein.
The practicality of this procedure stems from the ready availability of the precursors in a state of high enantiomeric purity, and from the absence of significant loss of stereochemical integrity not only in the sulfinylabv4and phosphiny13" carbanions, but also in the intermediate species present in the coupling reaction. The present synthetic method represents a dramatic departure from and improvement over former app r o a c h e ~ . ~ Our J ~ ~ method circumvents the deficiencies inherent in the earlier methods and furthermore guarantees the absolute configuration of the products. lo A typical experimental procedure follows. A stirred oxsolution of (+)-(R)-ethylmethylphenylphosphine ~ (c 0.83, CH3ide3" (2.51 g, 14.9 mmol), [ a I 2 ' +24" OH) (ca. 95% optically pure), in 60 ml of dry tetrahydrofuran was cooled to -78" and treated portionwise with n-butyllithium in hexane (16.4 mmol), under an inert atmosphere (N2), After 25 min, dry copper(I1) chloride (2.38 g, 17.7 mmol) was added. The mixture was kept at -78" for 15 min, allowed to warm ( 2 5 O ) , and saturated with oxygen for 10 min. After hydrolysis (10 % H2S04), the mixture was extracted (CHCI3), and the combined extracts were washed (dilute aqueous NH3, then H,O), dried (K2C03), and evaporated. Distillation (Kugelrohr, 150" (0.02 Torr)) removed 300 mg of impure starting material. Chromatography of the residue on silica gel afforded 2.0 g of crude 1,2bis(ethylphenylphosphiny1)ethane (l), upon elution with CHCL-EtOH. Treatment with acetone separated 85 mg (3 %) of meso-1. Recrystallization (acetoneCHC13) provided an analytical sample," mp 191-192", (3) (a) 0. Korpiun, R. A. Lewis, 3. Chickos, and I
